1. Field of the Invention
The present invention relates to a motor drive device comprising a converter for carrying out a power running operation for supplying power from a three-phase AC power supply to a three-phase AC motor and a power regenerative operation for returning energy generated by the three-phase AC motor to the three-phase AC power supply.
2. Related Art Statement
In a motor drive device driving a machine tool, an industrial machine, a robot and so on, a PWM (pulse width modulation) converter system, which converts a form of respective input currents to a substantially sinusoidal waveform by applying a PWM control carried out by power switching elements to AC/DC conversion for converting input AC power to DC power, is often used.
This PWM converter system has advantages in that harmonics included in the respective input currents to the PWM converter system can be reduced, in that respective output DC voltages from the PWM converter system can be changed, and so on. However, at the time of the power regenerative operation for returning energy generated by a motor to a power supply, such as the time of a deceleration control of the motor, heat generated by switching of the power switching elements in the PWM control is larger than in case of a power regeneration of a 120 degree current-carrying system, as stated below, and described in Japanese Examined Patent Publication (Kokai) No. 6-62584 (JP6-62584A), for example.
Therefore, conventionally, it is necessary to stop the PWM converter system in order to prevent overheating of the PWM converter system when the load of a converter used in the PWM converter system increases.
On the other hand, in the power regeneration of the 120 degree current-carrying system widely used in conventional power regeneration, only two power switching elements out of six power switching elements are turned on in order to generate respective power regenerative currents in a phase representing the maximum electric potential among three phases of the three-phase AC power supply and a phase representing the minimum electric potential among the three phases. The power switching elements used in this 120 degree current-carrying system generates less heat than in case of the PWM converter system, since the number of switching of the power switching elements used in the 120 degree current-carrying system is less than in case of the PWM converter system. However, the 120 degree current-carrying system has disadvantages in that there are large harmonics included in the respective input currents to the 120 degree current-carrying system resulting from respective pulse-like large currents, in that a sudden change of output DC voltage from the 120 degree current-carrying system affects a control of a motor used in the 120 degree current-carrying system and so on. Incidentally, Japanese Examined Patent Publication (Kokai) No. 8-228490 (JP8-228,490A) describes a converter which is operated in diode rectification mode at the time of the power running operation in order to eliminate a loss of switching and is controlled by a PWM signal only at the time of the power regenerative operation.
Therefore, the object of the present invention is to provide a motor drive device capable of continuing an operation even in the case of an excessive load of a converter in which the PWM control of the converter is carried out at the time of the power regenerative operation.